Long Term Evolution, also referred to as “LTE”, is a radio access technology being standardized by the 3rd Generation Partnership Project, hereinafter referred to as “3GPP”. In LTE, all services are supported through a packet switched domain. Downlink and uplink transmission in LTE use multiple access technologies—orthogonal frequency division multiple access, referred to as “OFDMA”, for the downlink, and single-carrier frequency division multiple access, referred to as “SC-FDMA” for the uplink.
FIG. 1 graphically represents an example subframe of an OFDMA/SC-FDMA radio signaling. In both OFDMA and SC-FDMA, a large number of closely spaced orthogonal subcarriers are transmitted in parallel. Thus, the signaling is defined by both frequency and time components.
In LTE, radio communication between a user equipment, hereinafter referred to as “UE” and an eNodeB is performed through defined radio frames. Each frame is 10 ms long and is divided into 10 subframes, each 1 ms long. Each subframe is further subdivided into two slots each with 0.5 ms duration. Thus, a transmitted signal in each slot or subframe is defined by a resource grid of a number of subcarriers in the frequency domain and a number of symbols in the time domain.
In FIG. 1, one downlink/uplink subframe, i.e., two slots, of LTE's radio resource is illustrated. The subframe is allocated in units of one or more physical resource blocks, hereinafter referred to as “PRB”. That is, the PRB is the smallest unit of radio resource assigned by the eNodeB for any UE. Depending on the configuration, each PRB spans a number of subcarriers in the frequency domain and spans a number of symbols, either OFDM or SC-FDMA, in the time domain. One symbol on one subcarrier is a resource element or simply “RE”.
In each PRB, there are 12 consecutive subcarriers in the frequency domain. With a normal spacing of 15 kHz between adjacent subcarriers, the frequency bandwidth of each PRB is 180 kHz. The PRB also spans 7 symbols in the time domain, i.e., the PRB spans one slot or 0.5 ms.
No dedicated channels are used in LTE to transport user data. Instead shared transport channel resources are used in both the downlink and the uplink. For the uplink, the uplink shared channel is controlled by a scheduler on the eNodeB that assigns different parts of the shared channel to different UEs for transmission of user data to the eNodeB. The uplink shared channel is mapped to the physical uplink shared channel, hereinafter referred to as “PUSCH”, on the downlink SC-FDMA subframe. The PUSCH is used primarily for data transport, and therefore, is designed to achieve high data rates.
The SC-FDMA subframe also includes the physical uplink control channel, hereinafter referred to as “PUCCH”. The PUCCH is used to carry uplink control information, or simply “UCI”, from the UEs to the eNodeB as will be explained further below.
FIG. 2 illustrates a simplified view of the SC-FDMA subframe. The view is simplified in that only the PUSCH and PUCCH resources are illustrated. It is recognized that in addition to the PUSCH and the PUCCH, the SC-FDMA subframe can also carry the physical random access channel, referred to as “PRACH” and the reference signal. But in general, the amount of the SC-FDMA subframe resources allocated to the PRACH and the reference signal is minor in relation to the PUSCH and the PUCCH. FIG. 2 reflects the observation that the SC-FDMA subframe is predominantly shared between the PUSCH and the PUCCH.
The PUSCH resource occupies the middle subcarriers of the SC-FDMA frequency spectrum. On the spectrum band edges, a control region is located on which the PUCCHs are transmitted. As mentioned above, the PUCCHs are allocated to allow the UEs to transfer the UCIs to the eNodeB. The UCI includes channel quality indication reports, scheduling requests, and ACK/NACK responses from UEs to previously scheduled downlink user data transmission from the eNodeB. The channel quality indication and the scheduling requests may also be referred to as “CQI” and “SR”, respectively.
To enable efficient resource utilization of the PUCCH, the SR, CQI, and ACK/NACK responses of several UEs are multiplexed on the PUCCH through code division multiplexing, also referred to as “CDM”. This allows several UEs to share one PRB of the PUCCH.
As noted previously and illustrated in FIG. 2, the uplink SC-FDMA subframe is primarily shared between the PUCCH and the PDCCH. That is, when a greater portion of the SC-FDMA subframe is allocated to the PUCCH, less is available for the PUSCH and vice versa.
Since the PUSCH carry user data, then to increase user data throughput, the PUSCH allocation should be maximized. This can be accomplished by an aggressive allocation of the PUCCH, e.g., assigning many CDM codes so that many UEs can simultaneously transmit the CQI reports, SRs, and ACK/NACK responses, etc., on the same time-frequency resource, i.e., share the same PRB of the PUCCH to minimize the PUCCH and correspondingly increase the PUSCH. However, many UEs simultaneously using the same PRB carries a risk of unacceptable error probability due to interferences. This in turn would decrease the system and/or user throughput and degrade overall system performance.
On the other hand, an unnecessarily cautious allocation—either through increasing the amount of resources allocated to the PUCCH or reducing the amount of signaling sent on the PUCCH—also carries risks. Increase in the of PUCCH allocation correspondingly reduces the uplink resources available for the PUSCH, which can lead to reduced data throughput. A reduction in the signaling amount can lead to a performance degradation due to delay caused by sparser SR opportunities, less accurate channel information due to sparser CQI reports, and restrictions on the scheduler due to less flexibility in the number of ACK/NACK responses.